Electrochemical cells have been used for detection of toxic gases since the 1970s in, for example, fixed location instrumentation for infrastructure (such as buildings and parking garages) and portable safety and inspection equipment used in transportation. For example, see Stetter, J. R., “Instrumentation to Monitor Chemical Exposure in the Synfuel Industry,” Annals American Conf. of Governmental and Industrial Hygienists, 11, 225-269, (1984). These sensors may be preferred in ambient monitoring applications because of their accuracy at low detection levels, selectivity, linearity, and power requirements. Industrial-grade electrochemical cells can cost the customer over $25 each and even several hundred dollars without any electronics, even when manufactured in high volumes. This cost can significantly increase the cost of gas monitors and detectors, and can leave manufacturers with few cost-effective options to create ultra-cheap, yet high performance gas detectors. For example, high quality, accurate devices for sensing carbon monoxide and triggering an alarm in the presence of excessive concentrations of carbon monoxide (CO) that may be hazardous to life or health are presently available for many industrial applications, but such devices are still too costly for use in most homes.
As a result, less expensive sensors with much lower performance are chosen to meet high volume consumer product cost goals, resulting in lower performance and a sacrifice of needed safety and health protection for the consumer. Additional consumer, medical, and industrial applications will be made available with a significant reduction in the cost and dimensions of electrochemical gas sensors. Other prior art gas sensors may use a liquid proton conductor where the outside surfaces of the sensing and counter electrodes of the sensor are coated by NAFION™ layers. NAFION™ is subject to freezing at 0° C., hindering operation of a sensor coated by NAFION™ at temperature of 0° C. and below. Further, the lifetime of these sensors can range from about 6-12 months due to rapid drying of the liquid electrolyte. Thus, the sensor requires maintenance due to liquid electrolyte evaporation, leakage, and/or corrosion. In addition, the sensors can have significant manufacturing costs and be relatively large, sometimes with large electrolyte or water reservoirs, which make integration of these sensors into modern equipment or small personal monitors difficult.
Another prior art gas sensor uses a design incorporating proton conductors, one type of electronically conductive metal catalyst for the sensing electrode, and a different type of electronically conductive metal catalyst for the counter electrode. This allows the sensing electrode to decompose a gas to produce protons and electrons, while the counter electrode exhibited no activity to decompose the gas. The result is a Nernst potential between the two electrodes, which can be used to detect a target gas. However, issues that could result from such a design include the gas reaction being carried out slowly or interfering reactions occurring on one or the other electrode surface. Additionally, the response signal could be weak. Further, the Nernst potential may be difficult to zero in clean air and the calibration is limited to about 59 mV per decade of concentration. Again poor electrolyte or electrode stability over time can degrade performance of a potentiometric gas sensor which often operate better at a high temperature.
Thus, a competitive electrochemical sensor that can cost less to manufacture in high volume and has high performance and small size, that would create a new opportunity for companies to develop low-cost gas detectors that could be manufactured in high volumes, thus making high accuracy detectors, for example, those that monitor and detect carbon monoxide and protect people and assets, much less expensive. This cost reduction, without loss in performance, could revolutionize and tremendously expand the use of gas detectors in their application, including home carbon monoxide monitors, automobile air quality, and building ventilation and controls. In addition, new applications would become possible, including safety organizations that may desire to inexpensively protect or monitor a large area from toxic gases like carbon monoxide, and universities or scientific/environmental organizations wanting to study toxic gas levels over large areas. In addition, an electrochemical sensor that also can be small can be used in cell-phones to enable worldwide networks of CO and other gas monitors.
The traditional porous, composite electrode is comprised of 10-40% polytetrafluoroethylene (PTFE) by weight and 60-90% catalyst by weight. The traditional electrode has possible residual dispersing, surfactants and thickening agents. These residual components are chemically inert and electrochemically inert. These electrodes are cured and/or sintered near the melting point of PTFE, typically 290-310 C. This requires printing on substrates such as porous PTFE that can withstand the PTFE cure temperatures. The PTFE serves as a binder to hold the catalyst particles together in a porous bed. It also serves as the hydrophobic portion of the composite bed electrode to provide a proper environment for a triple-phase boundary. This triple-phase boundary is desirous for gas-phase amperometric sensors.
While a variety of devices and techniques may exist for detecting gases, it is believed that no one prior to the inventors has made or used the inventive embodiments as described herein which have allowed the thin and tiny form factors and the low cost assembly achieved herein together with the high performance.